1. Field of the Invention
The present invention relates to a method for making a soft magnetic film for use in a magnetic head of a magnetic recording and reproducing apparatus such as a video cassette recorder (VCR) and an audio recording and reproducing apparatus.
2. Description of the Prior Art
In response to the requirement of high recording density in recent magnetic recording technology, much attention has been paid to the development of a magnetic head having a superior performance. In order to achieve a high recording density, it has been necessary to make the track width and the gap length of the magnetic head as narrow as possible and to manufacture a magnetic head from a core material of a soft magnetic film having a high magnetic flux density and a high magnetic permeability.
In order to satisfy the requirement mentioned above, various types of magnetic heads have been developed: One example as shown in FIGS. 9-10 is a magnetic head of a laminated-type head having a ring shape in which the core material is made of layers of a soft magnetic film 10 and an electric insulating film 11 laminated alternately and sandwiched between a pair of non-magnetic substrates 12. The core material forms a magnetic path. 13 denotes a glass material and 14 a magnetic gap.
Another example as shown in FIG. 11 is a magnetic head (referred to as an MIG head) in which the majority of the magnetic path is composed of ferrite material 15 and at the vicinity of the magnetic gap 16 easily saturated magnetically there is provided a soft magnetic film 17. 18 denotes a glass material and 19 an insulating film.
In the magnetic heads, the performance thereof relates closely to the material characteristics of the core material. In order to achieve a high recording density, it is necessary for the core material to have a high saturation magnetic flux density (related to mainly to the recording characteristics) and a high magnetic permeability (related mainly to the reproduction characteristics).
In connection with the requirement mentioned above, the core material for the above laminated-type head must be formed from a material having a high isotropic permeability. A core material practically used at present is Sendust film (Fe--Al--Si alloy) or a cobalt based amorphous alloy film. On the other hand, the core material for the MIG head is preferably made of a soft magnetic film having a high magnetic permeability with a uniaxial anisotropy in a plane. The core material practically used at present is Sendust film (Fe--Al--Si alloy) or a cobalt based amorphous alloy film. However, the Sendust film (Fe--Al--Si alloy) or cobalt based amorphous alloy film has a saturation magnetic flux density as low as about 1 T. Although effort has been directed to achievement of high recording density by using a recording medium having a high coercive force, the conventional core material has a limitation in the saturation magnetic flux density.
In view of the above situation, research and development have been directed to a soft magnetic film having a high saturation magnetic flux density and a high magnetic permeability. The core material under research is a film composed of (Fe,Co)--M--(N,C,B) alloy system (wherein M is at least one metal selected from the group consisting of Zr, Hf, Ti, Nb, and Ta), Fe--Co--B alloy alloy or Fe--N alloy alloy.
On the other hand, research on film formation methods has been carried out by using an electron beam evaporation method or sputtering method in connection with a soft magnetic material such as the Sendust alloy film or Permalloy film. Specifically, the magnetron sputtering method makes it possible to carry out the film formation at a high speed by improving on the usual disadvantage of the sputtering method which is a film formation rate lower by one order than the electron beam evaporation method. Further, recent progress in the film formation technology has produced new types of sputtering apparatus such as a carrousel type and an in-line type. The sputtering apparatus of the carrousel type is provided with a magnetron sputtering electrode in the shape of a rectangular flat plate and carries out the film formation while rotating a cylinder type substrate holder. The sputtering apparatus of in-line type is of a large scale and carries out the film formation while moving the substrate in parallel to a target. These sputtering apparatuses can execute the film formation at a high speed which is characteristic to the magnetron sputtering and obtain a uniform film thickness over a large area. As a result, a mass production of the soft magnetic film can be achieved by these sputtering apparatuses.
FIG. 18 is a schematic illustration of a conventional magnetron sputtering electrode provided with a target of a rectangular flat plate. A target 1 is adhered to a backing plate 5 with a soldering material such as indium and attached to an electrode body 6 thorough an O-ring vacuum seal. The target 1 has a magnetic circuit for the magnetron discharge formed at the back side thereof. The magnetic circuit forms a closed loop of lines of magnetic force 7 and at least a part of the lines of magnetic force 7 are arranged in parallel to each other at the surface of the target 1. As a result, there is formed, at the surface of the target 1, a magnetic field of a toroidal type having a closed tunnel shape. When the sputtering electrode attached to the rectangular flat plate target 1 is applied with a negative voltage through a DC or AC power source, the magnetron discharge is generated in the vicinity of the magnetic field of toroidal type having a closed tunnel shape where the electric field and the magnetic field cross each other. Then the target 1 starts sputtering to form a soft magnetic film on a substrate 4.
A core material known to have a high saturation magnetic flux density and a high magnetic permeability is a film composed of the (Fe,Co)--M--(N,C,B) alloy system (M is at least one metal selected from the group consisting of Zr, Hf, Ti, Nb, and Ta), Fe--Co--B alloy system or Fe--N alloy system and is made by a conventional magnetron sputtering method using a rectangular flat plate target or by a reactive sputtering using nitrogen gas for formation of a nitride film. In this case, it is necessary to sputter a thick target composed of a core material having a high saturation magnetic flux density such as (Fe,Co)--M--(N,C,B) alloy system (where M is at least one metal selected from the group consisting of Zr, Hf, Ti, Nb, and Ta), Fe--Co--B alloy system or Fe--N alloy system. The thick target prevents the magnetic flux from passing over the target surface. As a result, the magnetron discharge does not occur and sputtering is prevented.
Use of a thinner target permits the magnetron discharge and makes it possible to perform the sputtering. However, the conventional magnetron sputtering method causes the target to be eroded heterogeneously because the magnetic field produces an area to be sputtered (referred to as an erosion area hereinafter and shown by reference numeral 9 of FIG. 18) and an area having the sputtered particles re-adhered thereto. This causes the erosion area to change with the progress of the sputtering. As a result, the thickness distribution of the sputtered film changes, which causes the magnetic properties of the resultant film to change with the change in the thickness distribution. Therefore, there is a problem that the conventional magnetron sputtering method has a poorer efficiency of target utilization when the target is made thinner. This results in a problem that it is difficult to manufacture the magnetic film in a large scale.
On the other hand, as described above, the core material for the multilayer type of head requires a material having a high permeability and an isotropy in a plane. The core material for the MIG head mentioned above and a head of a main-pole driven magnetic head require a soft magnetic film having a high magnetic permeability to induce uniaxial anisotropy in a plane. Therefore, it is important to control the magnetic anisotropy.